Method of chromium plating aluminum



United States Patent METHOD OF CHRQMIUM PLATING ALUMINUM Application October 24, 1955 Serial No. 542,516

13 Claims. 01. 204-51 No Drawing.

This invention relates to the chromium plating of aluminum or aluminum base alloy parts or products to provide such articles with supplemental integrated surfaces having improved corrosion resistance.

More specifically the invention is directed to the application of chromium directly upon aluminum or aluminum base alloy articles by employing the method of electrodeposition. This application constitutes a continuation-in-part of my copending application Serial No. 454,621, filed on September 7, 1954, relating to the chromium plating of aluminum and which is now abandoned.

On the basis of the protective nature of a chromium plating coating on a much softer and weaker structural member made from aluminum, the combination produces an ideal product both light in Weight and one that has a protective covering. In its general aspects, the invention is concerned with the production of chromium plated structural elements or other articles made out of aluminum or aluminum base alloy metals to provide a protective covering thereon to resist physical deterioration and corrosive wear and tear.

A parallel consideration in this process contemplates the idea of chromium plating of aluminum or aluminum base alloy products and structural units for the purpose of providing decorative and corrosion resistant elements that are capable of taking and maintaining a high gloss luster finish. Such aluminum forms may be widely used for many purposes in varied installations such as decorative trim in the automotive field, appliance field, on building units as moldings, etc.

it is an object of the invention, therefore, to provide a plating method that deposits, in a minimum of time, a firmly adherent coating of ductile chromium directly upon an aluminum article to provide a chromium coating which resists cracking upon bending the covered article and which coating can be easily and readily polished or bufied to high luster.

It is another object of the invention to conduct the chromium plating process in such a manner as to reduce the tendency to fouling of the electrolytic bath employed with trivalent chromium and with such impurities that tend to accumulate therein and impair the continuous or efiicient use of the bath.

It is another object of this invention to carry out the plating process under controlled low temperature conditions, keeping the plating tank solution at a constant predetermined continued low temperature to provide a given hardness of corrosion resistant chromium plate coating on an aluminum article. The variation of the temperature within certain specific low temperature limits provides a range for a predetermined deposition of chromium bearing a definite degree of hardness.

It has been found that by continuously maintaining a selected low temperature within the plating bath that greatly improved predetermined results are obtained under simplified conditions of operation in the production 2,836,550 Patented May 27, 1958 of chromium plated aluminum. The deposited chromium adheres well to the aluminum stock and it does not crack upon bending the stock. The chromium coating is developed under the low temperature condition to become relatively soft and ductile having such a structural coating that can be easily polished. It has been also found that with the low temperature control plating that a much lower current density and a shorter period of plating time is required to produce an effective chromium coating directly upon aluminum under the low temperature conditions.

In carrying out the present process of the chromium plating of aluminum or aluminum base alloys, the tank electrolyte is prepared to normally contain chromic acid and a sulphate radical to constitute a plating bath that is then operated at a selected cold temperature below 51 Fahrenheit and preferably at a temperature selected from between 45 and 50 Fahrenheit and this temperature is constantly maintained during the plating cycle. This operating temperature condition consistently produces an ideal plating cycle causing the electrodeposition of a corrosion resistant chromium coating on the aluminum with the chromium having a hardness that is easily polishable to produce a high luster and a bright corrosion resistant surface. The chromium plate deposited at the temperature range designated is easily polished in a short interval of time, thus reducing polishing costs to a minimum and reducing rejects to a minimum because of the ease with which that operation can be performed.

When the plating operation is conducted at a tempera ture above 50 F., a harder chromium plate is deposited upon the aluminum and this immediately causes increased polishing requirements with a corresponding increase in time and costs. Such a harder plated surface is harder to polish creating a hazard and the polisher takes off more plate and will also frequently cut through the plate. it was also found that the time for depositing a comparable or equivalent thickness of chromium during a bath temperature ranging above 50 F. that this procedure increased the plating time considerably.

The temperature of the plating bath may be maintained at the constant low temperature level by suitable cooling coils, but it is preferred to use normal temperature controlled artificial refrigeration for forcing a refrigerant through a coil or coils located in the plating bath. It is also preferable to thermostatically control the cooling of the plating solution by maintaining the temperature of the bath Within the selected low temperature range, since the temperature normally tends to rise during the plating reaction. Current efiiciency has been found to be greatest at low temperatures, but uniformly eflicient results are obtained in regular continuous operation when the temperature of the plating bathis maintained within the preferred range. I

To illustrate the improvement effected by the method, the following comparative examples are given:

(1) Aluminum strips were plated with a solution containing 250 grams of chromic acid and 2.5 grams of sulphate radical per litre using .75 ampere per square inch at a temperature of 77 F. and a chromium coating having a thickness of .001 inch was produced in 1 hour and 45 minute Using the same bath and current density and a temperature of 35 F. produced a coating .001 inch thick in 35 minutes.

(2) Using the same plating bath at a current density of 2 amperes per square inch and a temperature of 131 F. a. coating .001 inch thick was'deposited in 1 hour. Using the same bath at a current density of 15 ampere per square inch and preventing the temperature from rising above 50 F. during a period of 5 minutes produced a coating .0001 inch thick. H 7 1 'ratio' much too' frequently.

X500 diameters. trend effects by standard statistical regression methods Where the temperature is not allowed to exceed 50 F. and by maintaining a currentdensity of 0.4 to 0.9 ampere per square inch, the time 'required'for a normal decorative chromium .coating, does not exceed minutes. The lower thetemperature the lower the current density required. For example, at 50 F. a current density of 0.5 ampere per square inch deposits a coating of approximately .0001 inch of chromium which is adherent, ductile and very easily polished. By maintaining the low temperature, the bath may be used almost indefinitely by merely renewing the chromicacid and sulphate radical. While the conductivity increases with chromic acid concentration, the current efliciency decreases and it is best to maintain a CrO concentration of 250 g./l.

Under past and present day plating methods that are conducted without controlled refrigeration to the low Using a plating tank containing 55 cubic feet of a refrigerated electrolyte and with a power source of 3000 7 V be utilized, thus reducing the overall cost of equipment.

'It is a further advantage of the present method that under the operating conditions of this low temperature bath; the equilibrium trivalent chromium concentration is zero, therefore eliminating the problems which arise when plating in the presence of trivalent chromium.

As a further means of evaluating the effect of temperature on the hardness of the chromium coating, a group of samples of aluminum extrusions coated at a variety of'temperatures were subjected to hardness tests using the Tukon hardness testing machine equipped with a a 136 Vickers diamond indenter and a load of 100 grams; The measurements of the impressions were made using a filar micrometer eyepiece at a magnification of The resulting data were examined for and a significant trend showing increasing hardness with increasing electrolytic bath temperatures 'was clearly established.

During the plating operation gases are given ofiand these tend to carry away some chromic acid which is lost. At-the low temperature range used in the present method, the .evolution of gas is much less violent with a 'corresponding reduction in thetendency to agitate the sludge in the tank which tends to accumulate inthe bath during continued operation. .This. avoids'pitting of the coating. After. normal cleaning of the aluminum articles to be plated,- it is common practice to dip them in a solution of zinc oxide in'caustic soda to assist the bond between the aluminum and the chromium coating. This tends tobring zinc and caustic soda into the plating bath which is. a solvent for zinc. The lower temperature used in the presentjmethod decreases the tendency to impair finishcoat. This obviouslyjreduces production costs of such chromium plated articlesc Moreover, the corrosive resistance of the coating formedat the low temperatures r '4 of 45 to 50 F. is much greater than coatings that are deposited at the normal higher plating temperatures on a nickel or copper base undercoating.

When the articles are plated at lower temperatures, as for example at about 35 F., they may be polished to a bright luster by a light bufiing, while at 50 polishing is effected by one pass under a normal polishing wheel rotating against the forward movement of the plated article and one pass with the article traveling with the rotation of the wheel. In comparison, under normal plating temperatures of F. it is economically and practically impossible to obtain the desired luster.

As a modification of the present plating process, the

solution of the plating tank can be varied and maintained at lower temperatures levels beneath the 45 to 50 F. range and down to the freezing point of the electrolyte to obtain selected softer chromium deposits on the aluminum articles. There appears to' be no definite lower limit for the operating temperature of the bath, but the operation of chromium plating aluminum articles cannot be carried on in a tank having a frozen electrolyte. g

It is, therefore, the freezing point of the tank solution that controls the low temperature limit of the modified process of chromium plating aluminum or aluminum base alloys under constant level low temperature conditions to obtain the kind of corrosion resistant chromium finish specified under the enhanced efficiency operation and under better solution control and maintenance.

What I claim is:

l. The method of chromium plating directly upon a metal selected from the group consisting ofaluminum and aluminum base alloy articles to deposit a continuous, consistent, highly ductile and easily buffed plate and one that remains crack free upon bending of the finished articles; which comprises providing an electrolytic bath consisting essentially of chromic acid and the sulphate radical, and maintaining the temperature of the bath at a substantially constant selected temperature level of 50 F;

throughout the plating cycleof operation for such articles.

bath between 451and 50 F. throughout'the plating cycle of operation for such articles. 7

3. The method of chromium plating of 7 aluminum articles to deposit a continuous, consistent, highly ductile and easily buffed plate and one that remains crack free upon bending of the finished articles; which comprises immersing the articles in an aqueous solution containing chromic acid and the sulphate radical, electrolyzing the solution under'a current density of 0.4 to 0.9 ampere per square inch, for not more than five minutes, and refrigerating the solution to maintain'the o'perating temperature of the solution not higher than 50 F. during the full.

extent of the required period of the complete plating cycle.

4. The method of chromium plating of aluminum articles to deposit a continuous, consistent, highly ductile and easily buffed plate and one that remains crack free upon bending of the finished articles; whichv comprises immersing the articles in an aqueous solution containing chromic acid andthe sulphate radical, electrolyzing the solution under a current density of0.4 to 0.9 ampere per square inch, for not more than five minutes, and refrigerating the solution .to maintain the operating tem-.

perature of the solution between 45 F. and 50 F. during V the full extent of the required period of the complete plating cycle. l 5. Themethod ofelectrodcpositing chromium directly assesses upon a metal selected from the group consisting of aluminum and aluminum base alloy products to deposit a continuous, consistent, highiy ductile and easily buffed plate and one that remains crack free upon bending of the finished products; which comprises providing an elec trolytic bath consisting essentially of chromic acid and the sulphate radical, immersing the articles to be plated in said aqueous solution of chromic acid and the sulphate radical, electrolyzing the solution under a current density of 0.4 to 0.9 ampere per square inch of surface to be plated and maintaining said solution at a maximum temperature not over 50 F. through controlled cooling means and throughout the entire cycle used to chromium plate said aluminum and aluminum base alloy products.

6. The method of eliminating imperfect chromium plating of a metal selected from the group consisting of aluminum and aluminum base alloys wherein said imperfections are normally characterized as pitting, gritty surfaces or lack of continuity of the chromium plated surface caused by excessive gas evolution and sludge agitation in the plating solution, which comprises providing an electrolytic solution consisting essentially of chromic acid and the sulphate radical, providing a refrigerating system for cooling said solution to stabilize the temperature of the solution at a selected constant and consistent cool plating temperature level below 51 Fahrenheit to prevent excessive gas evolution and sludge agitation, and electrolyzing the solution and maintaining said solution at said selected low temperature throughout the complete plating cycle of operation.

7. The method of eliminating the formation of unstable trivalent chromium in an electrolytic bath adapted for the chromium plating of a metal selected from the group consisting of aluminum and aluminum base alloys which comprises providing an electrolytic solution consisting essentially of chromic acid and the sulphate radical, pro viding a refrigeration system for said electrolytic solution to stabilize the temperature of the solution at a selected constant plating temperature level below 51 Fahrenheit to prevent the formation of said trivalent chromium, and maintaining the electrolytic solution at said selected low temperature level throughout the complete plating cycle of operation.

8. The method of chromium plating a metal selected from the group consisting of aluminum and aluminum base alloy articles which comprises providing an electrolytic bath consisting essentially of chromic acid and the sulphate radical, providing cooling means to reduce the temperature of the plating bath and the temperature of the articles to be plated to a selected constant low temperature level below 51 Fahrenheit, immersing said articles in said bath and establishing an electrolyzed bath, and maintaining the cold bath with the immersed cold articles at said stabilized temperature throughout the entire cycle of operation employed for the plating of said articles.

9. The method of chromuim plating directly upon aluminum articles which consists of providing a plating bath for said articles and stabilizing and maintaining the 69 temperature of said bath at a selected constant low temperature level under 50 Fahrenheit for the duration of the plating cycle.

10. The method of providing a given hardness of ductile chromium plate directly upon a metal selected from the group consisting of aluminum and aluminum base alloy articles which comprises providing an electrolytic bath for said articles and stabilizing and maintaining the temperature of said bath at a constant level low temperature selected from within a range of from 50 Fahrenheit and a lower limit temperature just above the freezing point of the electrolytic bath for the duration of the plating cycle, and providing a resultant predetermined hardness of plate as directly governed by the particular temperature selected.

11. The method of providing a decorative thickness of chromium deposit upon the surface of an aluminum article to produce a plate that is highly ductile, crack resistant and one that can be readily and easily polished or buffed to a high degree of luster with a minimum amount of effort; which comprises providing a plating bath consisting essentially of chromic acid and the sulphate radical; holding the temperature of said bath between 45 F. and 50 F. throughout the period of the plating cycle; and impressing a low density current of 0.4 to 0.9 ampere per square inch for a period of at least one minute but not in excess of five minutes; said operative conditions being carried on in the cool bath providing a plating cycle wherein the bath is not subjected to fouling through the formation of trivalent chromium or through gaseous agitation and wherein better adherence of the plate to the aluminum results from the more uniform, continuous and efiicient operation of the nonfouling and better current conducting plating bath.

12. The method of providing a long life chromium bath solution for plating aluminum with a chromium finish that is highly ductile, crack free on bending and one that is easily bufied; which comprises providing a bath consisting essentially of a mixture of chromic acid and the sulphate radical; and operating the bath at a given low temperature selected from between 35 F. and 50 F. with the use of a low density current for a period of not more than 5 minutes; said bath temperature being held constant at the selected level to counteract excessive gassing of said bath and to counteract the formation of trivalent chromium; and continuously supplying chromic acid and the sulphate radical to said bath to maintain the same mixture ratio concentration in said bath solution for chromium plating aluminum.

13. In the method recited in claim 12 wherein the concentration of the chromic acid in the bath is 250 grams/ liter, and the ratio of the chromic acid to the sulphate radical is between 100 to 1 and 200 to 1.

Work at al.: Transactions of the Electrochemical Society, vol. 59 (1931), pp. 429-435. 

5. THE METHOD OF ELECTRODEPOSITING CHROMIUM DIRECTLY UPON A METAL SELECTED FROM THE GROUP CONSISTING OF ALUMINUM AND ALUMINUM BASED ALLOY PRODUCTS TO DEPOSIT A CONTINUOUS, CONSISTENT, HIGHLY DUCTILE AND EASILY BUFFED PLATE AND ONE THAT REMAINS CRACK FREE UPON BENDING OF THE FINISHED PRODUCTS, WHICH COMPRISES PROVIDING AN ELECTROLYTIC BATH CONSISTING ESSENTIALLY OF CHROMIC ACID AND THE SULPHATE RADICAL, IMMERSING THE ARTICLES TO BE PLATED IN SAID AQUEOUS SOLUTION OF CHROMIC ACID AND THE SULPHATE RADICAL, ELECTROLYZING THE SOLUTION UNDER A CURRENT DENSITY OF 0.4 TO 0.9 AMPERE PER SQUARE INCH OF SURFASCE TO BE PLATED AND MAINTAINING SAID SOLUTION AT A MAXIMUM TEMPERATURE NOT OVER 50*F. THROUGH CONTROLLED COOLING MEANS AND THROUGHOUT THE ENTIRE CYCLE USED TO CHROMIUM PLATE SAID ALUMINUM AND ALUMINUM BASE ALLOY PRODUCTS. 